From the moment the seeds I’d started indoors sprouted in March, I was smitten. Beautiful, little vessels of life reaching toward the T-5 light. I rooted for them all to grow, grow, grow.
But I’ve gardened long enough now to know that their perfect leaves and stems face a tougher life ahead when I move them to their outdoor garden home. There will be occasional rain (though hopefully no more snow now), the risk of hail, strong and hot winds, intense high-altitude sunshine, and long dry spells. These little plant babies will face a beating as they grow up to produce the food and flowers my family has come to expect each gardening season.
Over the years I’ve learned useful strategies to help transition coddled indoor seedlings to the harsh world outdoors, and this year I’ve researched techniques to prepare plants that were started from seeds within the protective and controlled indoor environment so they can better withstand transplant shock and thrive in the garden. Below you’ll find the steps I take to toughen up my precious seedlings for the garden.
Harden off Indoor Seedlings Slowly to Build Resilience.
Plants are hardwired to survive. They grow so that they can reproduce and set seed, assuring their species’ survival. But, when started within indoor artificial environmental conditions, even the most stubborn plant species faces hurdles as they adjust to their harsh but natural growing environment. A “hardening off” process is your tough-love gift to indoor plant babies destined for greatness outside. This process of acclimating your indoor seedlings to the more unpredictable outdoor conditions in your garden before transplanting is the best way to help seedlings prepare for permanent life outside.
Seedlings need time to adjust to differences in temperature and air flow as well as intense sunlight, even more so in Colorado gardens where we have plenty of sun and gardens are usually growing at least a mile above sea level. Plant scientists have learned that plants develop a natural sunscreen that protects them from too much UV-B rays. Giving your seedlings slow but increasingly more exposure to sunshine will prepare them to withstand the long hours of light they’ll need once they are transplanted.
Begin the hardening off process with two to three hours of outdoor time in a mostly shady, sheltered location. During this process, move to watering the seedlings deeply every other day or every two days (depending on local humidity conditions). The seedlings may wilt temporarily from even indirect sun, but if you notice they are wilting from lack of water, be sure to water well to reduce one of the many factors stressing your seedlings. If you’ve been fertilizing your seedlings, don’t do so during this process.
During this first week, increase outdoor time to at least one hour each day, and continue protecting the seedlings from direct exposure to sun, wind, precipitation, and cold temperatures.
During the second and final week of hardening off indoor seedlings, start leaving them outside overnight to prepare for planting as long as nighttime temperatures are not below your particular plant species’ minimum temperature conditions.
Ease Transplanting Day Stress With Water and Nutrition.
On planting day, water each seedling in their pots before transplanting according to their needs into a prepared garden bed or container. Once in place, water them in their new home to give their roots a supportive space to dig in.
New to me this year, use a well-diluted fertilizer at the planting site of each seedling to speed up the growing process again and to alleviate inevitable transplant shock. (Update: I skipped the blood meal fertilizer last year, opting instead for some compost in and around the hole where I transplanted each seedling. The plants did as well as the year before, so I'm sticking with compost!)
Help Transplants Focus on Forging Strong Roots, Stems, and Branches.
While not essential to successful outdoor transplanting, if you notice any of your seedlings with flower buds, pinch off the buds so that they can focus their energy on developing strong roots, stems, and leaves for the months ahead when they will produce weighty fruit. This bud pinching is particularly useful for long-season crops like tomatoes and peppers with fruit heavy enough to snap stems if the plants have not had ample time to grow before setting fruit.
I’ve now planted hardened off Brassicas and some cool-season flowers using these strategies, and this weekend I’ll be moving the tomatoes, eggplant, and hot peppers from pots to raised beds. Most of the tomatoes and peppers are jonesing to flower, so I’ve busily pinched buds as soon as I notice them.
If transplanting time is nearer than you’d like this year, don’t worry that you’ve missed your window to harden off seedlings. The hardening off process can be condensed into one week. Any amount of time you find to help your seedlings adjust to outside life before transplanting will reduce transplant shock and increase their long-term productivity.